Fix exception-by-pointer leak; add missing test coverage

[dmccoystephenson]

Summary

• `Grid::getLocationByCoordinates`, `Grid::getLocation`, `Environment::getFirstEntity`, and `Environment::getEntity` were throwing `new std::runtime_error(...)` (a heap-allocated pointer) instead of the exception object itself; the corresponding catch site in `Environment::moveEntityInDirection` caught by pointer, so the allocated exception was never deleted. All four throw sites now throw by value, and the catch site now catches `const std::runtime_error&`.
• Direct test coverage was added for previously-untested public methods identified in Missing test coverage for several public methods #28: `Environment::setName`, `Environment::printInfo`, `Environment::addEntityToLocation`, `Grid::setSize`, `Grid::addLocation`, `Grid::getLocationByCoordinates`, `Location::getEntities()`, and `Location::getNumEntities()`.
• `Grid::removeLocation` was intentionally left without new test coverage in this PR — writing a direct test surfaced a pre-existing iterator-invalidation bug in that method (reliable undefined behavior, confirmed both in a plain build and under AddressSanitizer, not a clean assertion failure). That defect is filed separately as Grid::removeLocation invalidates its iterator after erase, causing undefined behavior #31 with a proposed fix, so a regression test can land alongside the fix rather than in this coverage-focused change. A comment was left in `tests.cpp` at the point where that test would otherwise live, and Missing test coverage for several public methods #28 was commented with this explanation.

Test plan

• `make` builds cleanly
• All 34 registered tests in `tests_executable` pass (exit 0)
• Regression for the leak fix confirmed empirically: with the fix reverted (`git stash`), a debug build compiled with `-fsanitize=address` and `ASAN_OPTIONS=detect_leaks=1` reports leaks originating from the four throw sites; with the fix restored, the same build and test run reports no leaks
• All newly-added tests pass; `Grid::removeLocation` intentionally not exercised by a new test (see above)

Closes #29
Closes #28

This PR description was drafted during a Gardener session (https://github.com/Stephenson-Software/gardener).

[dmccoystephenson]

Self-review rubric:

• Scope: PASS — every changed line is necessary for Exception-by-pointer pattern leaks the heap-allocated exception object #29 (throw-by-value fix, 4 sites + 1 catch) or Missing test coverage for several public methods #28 (new test coverage); no unrelated formatting/renames.
• Tests-new: PASS — every method added in this PR's scope (Environment::setName/printInfo/addEntityToLocation, Grid::setSize/addLocation/getLocationByCoordinates, Location::getEntities/getNumEntities) has a dedicated assert-based test.
• Tests-fix (empirical): PASS — the fix was stashed (git stash push -- src/environment.cpp src/grid.cpp), a debug build with -fsanitize=address + ASAN_OPTIONS=detect_leaks=1 was run, and LeakSanitizer reported 2124 bytes leaked across 72 allocations originating from the four throw sites (confirmed FAIL). The stash was popped and the same ASan build/run reported zero leaks (confirmed PASS).
• Sibling structure: PASS — new test functions follow the existing void test<Behavior>() / assert() / "Test N - ..." + "--- Success" pattern used by every neighboring test in tests.cpp.
• Sibling renames: N/A — no identifier renames in this PR.
• Docs: PASS — README's class table lists only Entity/Environment/Grid/Location with unchanged descriptions; no class or public surface changed, so no drift introduced.
• Issue resolution: PASS for Exception-by-pointer pattern leaks the heap-allocated exception object #29 (all 4 throw sites + catch site changed). PARTIAL-but-documented for Missing test coverage for several public methods #28: 8 of 9 named methods got direct tests; Grid::removeLocation was deliberately skipped after its test reliably reproduced undefined behavior (confirmed under a plain build and under ASan) rather than a clean assertion failure — filed as Grid::removeLocation invalidates its iterator after erase, causing undefined behavior #31, with a comment left in tests.cpp and a comment posted on Missing test coverage for several public methods #28 explaining the gap.
• Local test suite: PASS — make builds cleanly; all 34 registered tests print --- Success in order; process exits 0.

Repo-specific:

• Header/definition parity: N/A — no new methods or signature changes; only exception-throwing statements and a catch clause changed body-internally.
• Include guards: N/A — no new header files.
• Test registration: PASS — all 8 new test*() functions are called from main(); verified against main()'s call list.

Summary: one item is a deliberate partial per the "characterization, not change" rule — Grid::removeLocation's iterator-invalidation bug (#31) surfaced while writing its test, and the test-expansion policy says not to fix production code discovered incidentally in a coverage cycle. No other findings.

This comment was drafted during a Gardener session (https://github.com/Stephenson-Software/gardener).